Circumstances Around Rare Earthquake Lights Illuminated by New Study

New research published in the journal Seismological Research Letters offers insights into the mysterious flickers and flares of light occasionally seen before earthquakes.

So-called "earthquake lights" are indeed rare, but there is a documented history of the phenomenon dating back to the year 1600.

Robert Thériault, a geologist with Quebec's Ministry of Natural Resources, and his colleagues report that earthquake lights are more likely to occur on or near continental rift environments, where sub-vertical faults enable stress-induced electrical currents to flow rapidly to the surface.

To draw their conclusion, the researchers analyzed 65 of the best documented instances of earthquake lights dating back more than four centuries in Europe and North America. Eighty-five percent of the reported of earthquake lights were on or near a rift and 97 percent of the time the phenomenon occurred along a sub-vertical fault such as a rift, graben, strike-slip or transform fault. The researchers note the peculiarity of this data, as these sub-vertical faults are associated with just 5 percent of the Earth's seismic activity, but are connected to nearly all of the documented instances of earthquake lights.

"The numbers are striking and unexpected," said Thériault. "We don't know quite yet why more earthquake light events are related to rift environments than other types of faults, but unlike other faults that may dip at a 30-35 degree angle, such as in subduction zones, sub-vertical faults characterize the rift environments in these cases."

The earthquakes that triggered the flashes of light ranged in magnitude from 3.6 to 9.2, but 80 percent of the quakes in the study were greater than magnitude 5.0.

The earthquake lights also varied in size, shape and duration, but were typically in the form of what the researchers called "globular luminous masses" or in flame-like bands of light emanating from the ground.

Among the episodes of earthquake lights included in the study were:

• The 2009 L'Aquila, Italy earthquake, where people reportedly saw 10-centimeter high flames of light flickering above a stone-paved avenue in the town's center.
• A Nov. 12, 1988 earthquake along the St. Lawrence River near the city of Quebec, where there were reports of a bright purple-pink globe of light that moved through the sky 11 days before the quake.
• And in 1906, about 100 kilometers northwest of San Francisco, people saw streams of light running along the ground two nights preceding that region's great earthquake.

Earthquake lights were also captured by security cameras in 2007 before the magnitude 8.0 earthquake in Pisco, Peru, which along with seismic data, allowed researchers to pinpoint the exact timing and location of the mysterious lights.

Thériault said that the presence of earthquake lights could be used as an indicator of a coming earthquake, according to The Smithsonian blog.

"If you see visible lights in the sky, and you live in an earthquake-prone area, they might be an early-warning sign that an earthquake is approaching," Thériault said.